Power transformer rating?

[q2audio]

Hey guys,

I have a "found" power transformer that I want to use to power a 24x4 Soundcraft Delta console - at least the +/-17VDC part. The original supply (CPS 150, it's long gone) was rated at 1.5A per output. Is there a way to guess at the current capability of an unknown transformer by the size of the secondary wiring? Are there any handy 'rules of thumb'??

What I'm really doing: replacing the transformer in a Power-One linear 24V 2.4A supply. This supply has about +/- 39V DC feeding the regulators (pass transistors) - I need +/- 17V out so I'm throwing away quite a lot of voltage and I would like to lower this. The transformer I have is 40V CT with what looks like quite heavy wire making up the secondary. Just to guess it is probably about 24ga (solid). I can't remember exactly what I took this thing out of... some sort of consumer audio product, possibly a small power amp or a really BIG (old!) CD player. It does have a copper band around it. (Mmmmm, copper band)

Thanks!

Tim

 

[PRR]

VERY-VERY rough: 5 Watts per pound. This ain't right but will sort the 10-watt from the 100-Watt.

Then put a resistor on the secondary to load it to almost the estimate, and see how much it drops from no-load to load. Very small transformers normally drop 20% at full rated load. So a 40VAC no-load 0.25A transformer loaded with 40V/0.25A= 160Ω 10Watt resistor will show 40V*0.80= a bit over 32VAC (it would take 32V/0.25A= 128Ω to load it fully). Up in the 50 Watt range the rating may be taken at 10%: load the "40VAC" transformer to 40V*0.90= 36V and that's your current rating. Up above 100 Watts the safe long-term current rating may be at 5% drop.

Remember that the AC Amps of the transformer is typically twice the DC load current. Your 1.5A DC load needs a 3A AC transformer. To get a solid +/-17V or 34V DC you want well over 2*20VDC on load, plus a few volts for ripple and rectifier, 48VDC, which needs 34V AC at 3Amps AC.

So the smoke-test is a 34V/3A= 11Ω 100 Watt power resistor across the "40VAC" winding. If the drop is not much over 10%, leave it cook on a flame-proof board outside for an hour. If it does not burn your finger, is holding its voltage, it should be fine.

 

[caps]

PRR,

You dont have to answer this, but Im just curious as to what your background is.

Your breadth of knowledge on all things discussed here is astonishing.

 

[Kev]

[quote author="PRR"]VERY-VERY rough: 5 Watts per pound. [/quote]

that's the way my mentor would do it

pick up a transformer in one hand, look skyward to the larger power
then look me in the eye ... make a face ... and say .....
:roll: 60 VA
pass me the next one.



funny old fella ... but I learnt much from him

 

[PRR]

> "5 Watts per pound" --- that's the way my mentor would do it

It is how my one greatgrandfather sold ham, and interestingly it is how my other grandfather sold the airplanes his factory made: he said you could price an aircraft close-enough just by knowing how much it weighed.